Your card is running into its throttle point. That chart with max temperatures is all fun and good, but if Nvidia cards (ever from Kepler onwards, so since 2012) run past 80 C they will start running at considerably lower clocks unless you've done some specific core overclocking, and they will try hard to keep their temperature below 80 C.
The best temperatures for Pascal are below 50 C. That is impossible on air cooling, but with a regular blower style cooler Pascal really shouldn't get any hotter than 75 C - ideally, you'd want to shoot for 70 C tops to get the best performance out of your card. Go lower and you can clock a bit higher, but you'll only get that if you OC the card yourself.
FWIW, I would strongly suggest you find an alternative for the sound card of yours. Unless you're an audio enthusiast, most recent motherboard's onboard audio these days is more than OK.
Also next time you buy a GPU, avoid the blower style / reference coolers. They are absolute junk unless you have a really, really small case with little to no airflow (and as you can see, even then they aren't much good for your temperatures). Somehow EVGA manages to do worse on that reference design than Nvidia itself, as well. I've had an EVGA GTX 660 SC with a blower and it was similarly crappy.